Heating particulate material

ABSTRACT

Heating particulate material by means of radiofrequency induced plasma in a stream of gas by introducing the particulate material into the downstream half of the plasma through a feed tube which terminates within the downstream half of the volume of plasma.

United States Patent 1 72 Inventors Peter Compton MacDonald Gloucester; Arthur Leonard Riley, Fairfield, both of, England [21 I Appl. No 743,675

[22] Filed July 10, 1968 145] Patented July 27, 1971 [73 Assignee British Titan Products Company Limited Durham, England [32] Priority July '1 1, 1967 [33] Great Britain [54] HEATING PARTICULATE MATERIAL 27 Claims, 4 Drawing Figs.

[52] US. Cl. 219/76,

[51] lnt.-Cl H051) 5/00, 523k 9/04 [50] Field of Search 219/76, 10.53,10.57, 10.43, 10.49, 10.51, 10.41, 121, 121

[56] References Cited UNITED STATES PATENTS 3,277,265 10/1966 Rebouex 219/1049 3,296,410 1/1967 Hedger 219/76 3,429,665 2/1969 Evans et a1 Primary ExaminerJ. V. Truhe Assistant Examiner-L. 1-1. Bender Attorney-Birch, Swindler, McKie and Beckett ABSTRACT: Heating particulate material by means of radiofrequency induced plasma in a stream of gas by introducing the particulate material into the downstream half of the plasma through a feed tube which tenninates within the downstream half of the volume of plasma.

PATENTEDJULZY I97| zvzmoas PETER c MucDONALD ARTHUR L. RILEY BY AM, Law, an {l q/6156 ATTORNEYS HEATING PARTICULATE MATERIAL BACKGROUND OF THE INVENTION This invention relates to a method for heating particulate material and particularly to heating of such materials by the use of a plasma.

The production of plasmas by means of induction heating of a gas from aradiofrequency source is known and such plasmas have a high temperature which can be used to heat particulate material introduced into the gas stream containing the plasma. However, extreme difficulties are experienced in performing this heating operation, especially in diatomic gases, since it has proved difficult to introduce the solid material into the plasma ball (as it is so called) without extinguishing the plasma or without causing excessive deposition or growth of the solid material on the probe through which the solid is introduced.

SUMMARY OF THE INVENTION According to the present invention a method of heating particulate material comprises establishing and maintaining a volume of plasma in a gas-confining tube by a radiofrequency induced electric discharge in a stream of gas in said tube and introducing the particulate material to be heated into the volume of plasma by means ofa feed tube terminating with the volume ofplasma.

The method of the present invention is particularly useful for the heating of particulate material in the form of a powder in diatomic gases or other gases in which a plasma ball or volume of electric discharge is generated by the application of radiofrequency induction heating. It has been found that when the particulate material to be heated is introduced into the plasma ball, preferably the lower half or tail of the plasma, the material is heated in the ball to the desired temperature and growth on the feed tube or probe through which the material is introduced into the plasma ball or wall of the gas-confining tube is reduced or substantially prevented or reaches an acceptable equilibrium value.

Usually the plasma is produced and maintained in a gas stream flowing vertically downwards but if desired the gas stream can be angled to the vertical or can be horizontal. When the gas stream is angled then preferably it is at an angle of from 45 to 75 to the vertical.

By the term lower half, when the longitudinal axis of the volume of plasma is horizontal, is meant that half of the volume remote from the entry of the gas or gases forming the plasma.

The gas-confining tube is one having heat resistant walls, for example, of silica and the tube can be mounted so that it has its longitudinal axis vertical or horizontal or at an angle to the vertical. The axis of the tube usually will also be the axis of the plasma. The induced electric discharge in the gas stream is produced preferably from a coil of electrically conducting material surrounding the tube in the area where it is desired to generate the plasma ball and the coil can be, for example,.a coil of copper tubing. The gas stream passes along the axis of the coil which is the axis also of the confining tube and of the plasma ball. The ends of the electrically conducting material are connected to a source of oscillating current of the appropriate radiofrequency, preferably one in the range 200 kilocycles to 20 megacycles and particularly within the range of l megacycle to megacycles. The power input to the coil should be sufficient to maintain a plasma at the required energy level and to effect the desired heating of the particulate material introduced therein.

,By suitable techniques the application of the radiofrequenc'y electric current to the coil causes in the flowing gas stream a volume of electric discharge known as a plasma ball which is elongated in the direction of flow of the gas stream. An elongated portion, or the tail portion, extends beyond the lower turn of the coil and preferably it is into this portion of the plasma hall'that the particulate material is introduced.

Generally in such induction heating a liquid coolant, for example water, is circulated through the coil carrying the oscillating current. If desired, this may be replaced by a gas, for example, the gas which is to be heated to form a plasma and in this manner the gas may be preheated before passing into the confining tube. It is preferable, however, to circulate a liquid coolant through the coil.

It is also advantageous to cool the outside of the gas-confining tube and this may be accomplished either by flowing a gaseous coolant, for example air, on to the exterior of the tube or, if desired, by providing the walls of the tube with a jacket and circulating a liquid coolant through thisjacket.

It may be desired to perform the method of the present invention in such a manner that the gas is introduced into the confining tube to provide a helical flow around the plasma ball or alternatively the gas may be introduced in laminar flow down the interior walls of the confining tube thereby forming a sheath between theplasma and the confining tube wall. If desired, a mixture of both types of flow may be used, Generally, the flow pattern is so chosen to minimize deleterious growth of solid on the confining tube walls.

The number of turns in the coil used to induce the'plasma may vary, for example, there may be from 3-10 turns, preferably from 4-9 turns.

The particulate material to be heated is fed into the plasma volume through a feed tube or probe which terminates within the volume of plasma. The feed tube may be mounted parallel to the axis of the volume of plasma and the gas-confining tube or may be axially aligned with the axis of the volume and the gas-confining tubeor may be mounted at an angle to the axis of the volume and the tube. A volume of plasma usually has a central core portion or low energy area and preferably the feed tube terminates within this central core portion.

Preferably the feed tube terminates at a point in the lower half of the volume of plasma.

The diameter of the feed tube will depend'on the desired rate of feeding the particulate solid material and also on the particular gas used, the plasma power and frequency of operation. For instance when the gas is oxygen then it is desirable to terminate the feed tube within the core of the plasma and to reduce the diameter of the feed tube within the volume of plasma to about 0.220 inch.

Usually the feed tube or probe is cooled during use, for example by water, to prevent excessive heating of the tube by the volume of plasma. The residence time of the particulate material in the region of the plasma in the gas-confining tube can be varied by variation of the point of termination of the feed tube.

The efficiency of the method of the invention when the particulate material is fed axially into the volume of plasma can be improved by creating a low pressure region at the head of the plasma. For instance, this can be achieved by applying vacuum to this region through a pipe concentric with the feed tube and terminating in the position of the desired region of low pressure. The presence of this low pressure region gives rise to countercurrent gas flow along the axis of the plasma which carries all or some of the particulate material, depending on the position of the end of the feed tube relative to the top of the volume of plasma, back above the plasma and down the outside of the volume of plasma. This increases the residence time of the particulate material within the plasma region. The optimum position of the end of the feed tube using this modification is just below the first turn from the top of the coil of electrically conducting material.

The method of the invention can be used to heat a wide variety of powered particulate materials, e.g. metal powders, mineral powders, powdered titanium ores, for example ilmenite and rutile, China clay and bentonite. It is desirable that the material to be heated should be in a finely divided form, for example, the material should have mean particle size in the range from 30-1000 microns, preferably 30--500 microns and more particularly in the range from l00-200 microns. It is also desirable that the material to-be heated should be supplied ata controlled and preferably constant rate if optimum results are to be obtained. The material can be introduced into the volume of plasma in the form of a suspension in a gas stream which gas can be any suitable gas, for example nitrogen or argon.

One device which has been found very suitable for the supply of finely divided titaniferous material in the form of a gaseous suspension, particularly as a lean" gaseous suspension, is that described and claimed in our U.S. Pat. No. 3,412,892. By selection of the appropriate dimensions and conditions of operation of such a device a quantity of, for example, titaniferous material can be supplied at a constant rate in the form of a suspension in a stable carrier gas over a prolonged period of time. Normally the material is mineral rutilc and this can be supplied at a rate of up to It) grams/minute to a plasma generated in an oxygen gas.

The plasma can be generated in any gas or gaseous mixture such as monatomic or diatomic gases. Examples of gases are argon, helium, neon, krypton, carbon dioxide, oxygen, chlorine and the oxides of nitrogen. Preferably the gas is oxygen. When argon is used then it is preferably used as a mixture with oxygen.

The method of the present invention is particularly useful for the production of titanium dioxide from titaniferous material such as mineral rutile.

The production of finely divided titanium dioxide according to the present invention can be by the method described and claimed in our U.S. Pat. No. 3,429,665 which described the vaporization ofa solid material containing titanium dioxide by contacting it with a gas heated to a temperature above the boiling point of titanium dioxide and then condensing, preferably selectively, the finely divided titanium dioxide from the vaporized material.

The titanium dioxide produced can be collected in a suitable apparatus such as a sock filter and removed. Alternatively, the derived finely divided titanium dioxide produced after the initial fractional condensation can be recovered by electrostatic precipitation or by cyclones.

If rutile titanium dioxide pigment is desired then a rutilising agent can be introduced into the plasma with the titaniferous material and such agents are an aluminum trihalide or a zirconium tetrahalide particularly the chlorides. Alternatively, the rutilising agent may be introduced into the gas stream at a point beyond the plasma and before the condensation of the titanium dioxide is complete. It is also observed that slower cooling of the gaseous titanium dioxide produced in the plasma increases the proportion of rutile titanium dioxide in the product.

When the method is used to produce pigmentary titanium dioxide then the condensed product may be subjected to known treatments, for example, milling, hydroclassification and/or wet coating, if desired.

BRIEF DESCRIPTION OF THE DRAWINGS Various forms of the apparatus for operating the method of the present invention will now be described by way of Example only with reference to the accompanying drawings in DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown the apparatus of FIG. 1 consists of a silica tube I having an external diameter of 1.5 inches and an internal diameter of 1.4 inches, mounted so that it is inclined at an angle of to the vertical. The upper end of the tube I is provided with a gas distribution head 2 and a gas inlet 3. Positioned approximately 4 inches from the gas distribution head 2 is a coil 4 of copper tube. The coil 4 has an internal diameter of 2 inches and external diameter of 2.4 inches. The coil 5 has five turns enclosing a length of 2 inches of the tube 1 and is connected to a source of radiofrequency electric oscillatory current (not shown). Means are also connected to the coil 4 to pass a flow of water through the coil.

The tube I is provided with a sidearm 6 formed of silica tube extending vertically from the lower end of the tube 1. The sidearm 6 has an internal diameter of 0.375 inch and has located within it a hollow brass probe 7 sealed by means of a rubber sleeve 8. The probe 7 is provided with inlet means 9 and outlet means 10 to permit the circulation of cold water to cool the probe 7.

The central passageway I] of the probe 7 permits the introduction of the powder into the plasma and to achieve this the probe 7 is extended within the tube 1.

When in use and by suitable techniques the application of a radiofrequency current to the coil 4 induces a volume of plasma indicated at I2 into which the powder is fed.

The apparatus also includes when in use a tube member extending beyond the lower end of the tube 1 and a sock filter to collect the products of the heating operation.

Alternative forms of performing the method of the present invention using a gas-confining tube mounted vertically are illustrated in FIGS. 3 and 4. Basically the apparatus consists ofa gas-confining tube 20 through which the desired gas is passed vertically downward. Surrounding the tube 20 for a portion of its length is a coil 21 connectable to a source of oscillatory electric current to maintain a plasma volume 22 within the gas-confining tube 20.

The effect passing a particulate material such as mineral rutile through a feed tube terminating in the position indicated in FIGS. 3 and 4 into a volume of plasma within a gas-confining tube 20 are shown.

FIG. 3 illustrates the effect of terminating the feed tube 23 in the central core of the plasma 22 the buildup which occurs on the walls of the tube is shown at 24. The feed tube is mounted on the axis of the gas-confining tube 20 and it is found that a substantial proportion of the particulate material fed into the volume of plasma is heated whilst a small buildup does occur as shown at 24. Nevertheless, equilibrium conditions are achieved rapidly and further buildup of thickness does not occur.

FIG. 4 illustrates the effect of introducing the particulate material into the core of the plasma 22 but using the feed tube 20 mounted at an angle to the vertical and passing through the wall of the gas-confining tube 20. Some buildup on the wall surface opposite to the end of the feed tube 23 does occur up to an equilibrium thickness but a large proportion of the particles is heated by the volume of plasma.

FIG. 2 illustrates the effect of feeding particulate material into a gas-confining tube in which a volume of plasma 22 is maintained with the feed tube 23 terminating above the volume of plasma. Here, excessive and unacceptable buildup of particulate material occurs on the walls of the tube at the uppermost end of the volume of plasma and the efficiency of the heating is considerably reduced. It is the arrangement of FIG. 2 over which this invention is an improvement.

The following Examples illustrate the invention:

Example I The apparatus shown in FIG. 1 of the drawings and described previously was used.

Oxygen gas at a rate of 50 liters per minute was fed to the gas distribution head and fed through the silica tube 1. A radiofrequency oscillating current of 2.5 megacycles per second was passed through the coil to generate l2 kilowatts of power in the gas in the plasma ball which had been initiated previously. Cooling water was passed through the coil, 2.5 grams per minute of mineral rutile having a size of from 124 to 150p. was fed through the probe into the plasma from a vibratory feeder. It was found that from to percent of the feed was vaporized, 5 percent changed in form to spheroids and the rest sintered onto the walls of the vessel used to collect the product.

For comparison a similar amount of mineral rutile was fed as a stream into a plasma tube mounted vertically so that the rutile entered the top of the tube substantially as shown in FIG. 2. A plasma was maintained in the gas stream which was a mixture of argon and oxygen containing 40 percent of oxygen by the application of a radiofrequency current of 2.5 megacycles per second inducing 8 kilowatts of power in the plasma ball. it was found that only l percent of the rutile was vaporized, 40 percent changed in form and the remainder unaffected.

It will be seen that by use of the method of the invention a larger amount of rutile is treated and that a pure oxygen plasma can be used. With the previous vertically aligned plasmas the maximum amount of oxygen that could be tolerated was 40 percent in an oxygen/argon mixture.

Example 2 An apparatus to produce a plasma was employed according to the arrangement in FIG. 6 of the accompanying drawings in which the feed tube terminated in the lower portion or tail of the plasma and was aligned axially with the gas-confining tube.

Chlorine gas was passed through the gas-confining tube at 30 liters/minute and a plasma initiated in the gas and maintained by the passage of oscillatory current through the coil 21 to give a plasma having a power of l9.9 kilowatts.

Mineral rutile of average particle size, 12411. to 150 was fed through the feed tube at a rate of 2 grams/minute carried by a carrier gas which was argon flowing at a rate of liters/minute. The material was vaporized giving a high proportion of anatase titanium dioxide on condensation. Some buildup occurred as shown in FIG. 3 but this soon reached equilibrium conditions when further buildup did not occur.

Example 3 The arrangement shown in FIG. 4 was used but in this case the gas in which the plasma was generated and maintained was oxygen flowing at 74 liters/minute. The plasma was maintained at a power of 14 kilowatts and mineral rutile fed through the feet tube at a rate of flow 2-5 grams/minute with the carrier gas being oxygen flowing at 4 liters/minute.

Again, a large proportion of the product was vaporized and recovered as solid anatase titanium dioxide.

The experiment was repeated four times using different particulate materials fed into the plasma through the feedpipe and the experimental details are given in the following Table.

The ilmenite had a particle size of from 12411. to 150p Again, the mineral rutile after passing through the plasma was vaporized to the extent of from 75-80 percent producing a spheroidal product having a diameter of approximately 0.2 p. of which 97 percent by weight was anatase titanium dioxide. The bentonite was wholly vaporized and formed into spher'oids having a particle diameter of 0.04;z. China Clay spoils were wholly vaporized and fonned into spheroids having a diameter of approximately 0.05 The ilmenite was vaporized to the extent of from 7580 percent, with the product being spheroidal particles having a diameter of approximately 0.02 consisting mainly of iron titanate.

Example 4 Two experiments were performed denoted as A and B using apparatus substantially as described in FIG. 3, with, in Experiment A the feed tube terminating level with the bottom turn of the work coil and in Experiment B the feed tube terminating in the tail of the plasma.

The plasma was maintained in oxygen flowing at a rate of 50 liters/minute in each Experiments A and B. The plasma which was maintained in the gas had a power of 23.5 kilowatts in Experiment A and 19.4 kilowatts in Experiment B. Mineral rutile was fed through the feed tube in and amount of 56 grams/minute in Experiment A carried by a carrier gas being oxygen flowing at a rate of 2% liters/minute. In Experiment B, a mixture of mineral rutile and sand in equal weights was fed at a rate of 5 grams/minute with oxygen as a carrier gas flowing at 4 liters/minute.

In Experiment A the mineral rutile was vaporized and recovered as anatase titanium dioxide in a substantial proportion. In Experiment B the mineral rutile was vaporized whilst silica sand was aggregated.

In each of the above Experiments A and B the feed tube was water cooled and some buildup on the confining tube did occur as indicated in FIG. 3 of the drawings but this soon reached equilibrium conditions when further buildup did not take place.

What 1 claim is:

l. A method of heating particulate material which comprises establishing and maintaining a volume of plasma in a stream of gas in a gas-confining tube by passing a radiofrequency (induced) electric current through a coil of electrically conductive material mounted around (discharge in a stream of gas in) said tube and introducing the particulate material to be heated into the downstream half of the volume of plasma in said gas by means of a feed tube terminating within the downstream half of the volume of plasma.

2. A method according to claim 1 in which the gas-confining tube is mounted with its longitudinal axis vertical.

3. A method according to claim 1 in which the gas-confining tube is mounted with its longitudinal axis horizontal.

4. A method according to claim 1 in which the gas-confining tube is mounted with its longitudinal axis at an angle between the vertical and the horizontal.

5. A method according to claim 4 in which the gas-confining tube is mounted with its longitudinal axis at an angle of from 45-75 to the vertical.

6. A method according to claim 1 in which the electrically conducting material is connected to a source of oscillating current having a frequency in the range 200 kilocycles to 20 megacycles.

7. A method according to claim 6 in which the electric current has a frequency within the range 1 megacycle to 10 megacycles.

8. A method according to claim 1 wherein the coil comprises from 3- l0 turns.

9. A method according to claim 8 wherein the coil has from 4-9 turns.

10. A method according to claim 1 in which the feed tube is mounted parallel to the axis of the gas-confining tube.

11. A method according to claim 1 in which the feed tube is mounted so that its axis is axially aligned with the axis of the gas-confining tube.

12. A method according to claim 1 in which the feed tube is mounted at an angle to the axis of the gas-confining tube.

13. A method according to claim 1 wherein the feed tube terminates in the central core portion of the volume of plasma.

14. A method according to claim 1 wherein the feed tube is cooled during use 15. A method according to claim 1 wherein the particulate material to be heated has a particle size in the range from 30- 1000 1..

16. A method according to claim 15 wherein the particle size is from 30-500u.

17. A method according to claim 16 wherein the particle size is from l00-200a.

18. A method according to claim 1 wherein the particulate material is supplied through the feed tube at a controlled rate.

19. A method according to claim 18 wherein the particulate material is supplied through the feed tube at a constant rate.

20. A method according to claim I wherein the particulate material is introduced into the volume of plasma in a form ol'a suspension in a gas stream.

25. A method according to claim 23 wherein a rutilising agent is introduced into the gas stream at a point beyond the volume of plasma and before condensation of titanium dioxide thus produced is complete.

26. A method according to claim 24 wherein the rutilising agent is an aluminum trihalide or a zirconium tetrahalide.

27. A method according to claim 1 wherein the feed tube terminates within the central core of the downstream half of the volume of plasma. 

1. A method of heating particulate material which comprises establishing and maintaining a volume of plasma in a stream of gas in a gas-confining tube by passing a radiofrequency (induced) electric current through a coil of electrically conductive material mounted around (discharge in a stream of gas in) said tube and introducing the particulate material to be heated into the downstream half of the volume of plasma in said gas by means of a feed tube terminating within the downstream half of the volume of plasma.
 2. A method according to claim 1 in which the gas-confining tube is mounted with its longitudinal axis vertical.
 3. A method according to claim 1 in which the gas-confining tube is mounted with its longitudinal axis horizontal.
 4. A method according to claim 1 in which the gas-confining tube is mounted with its longitudinal axis at an angle between the vertical and the horizontal.
 5. A method according to claim 4 in which the gas-confining tube is mounted with its longitudinal axis at an angle of from 45-75* to the vertical.
 6. A method according to claim 1 in which the electrically conducting material is connected to a source of oscillating current having a frequency in the range 200 kilocycles to 20 megacycles.
 7. A method according to claim 6 in which the electric current has a frequency within the range 1 megacycle to 10 megacycles.
 8. A method according to claim 1 wherein the coil comprises from 3-10 turns.
 9. A method according to claim 8 wherein the coil has from 4-9 turns.
 10. A method according to claim 1 in which the feed tube is mounted parallel to the axis of the gas-confining tube.
 11. A method according to claim 1 in which the feed tube is mounted so that its axis is axially aligned with the axis Of the gas-confining tube.
 12. A method according to claim 1 in which the feed tube is mounted at an angle to the axis of the gas-confining tube.
 13. A method according to claim 1 wherein the feed tube terminates in the central core portion of the volume of plasma.
 14. A method according to claim 1 wherein the feed tube is cooled during use.
 15. A method according to claim 1 wherein the particulate material to be heated has a particle size in the range from 30-1000 Mu .
 16. A method according to claim 15 wherein the particle size is from 30-500 Mu .
 17. A method according to claim 16 wherein the particle size is from 100-200 Mu .
 18. A method according to claim 1 wherein the particulate material is supplied through the feed tube at a controlled rate.
 19. A method according to claim 18 wherein the particulate material is supplied through the feed tube at a constant rate.
 20. A method according to claim 1 wherein the particulate material is introduced into the volume of plasma in a form of a suspension in a gas stream.
 21. A method according to claim 1 wherein the stream of gas in said tube is argon, helium, neon, krypton, carbon dioxide, oxygen, chlorine or an oxide of nitrogen.
 22. A method according to claim 1 wherein the particulate material to be heated is a titaniferous material.
 23. A method according to claim 22 wherein the titaniferous material is mineral rutile.
 24. A method according to claim 23 wherein a rutilising agent is introduced into the volume of plasma with the titaniferous material.
 25. A method according to claim 23 wherein a rutilising agent is introduced into the gas stream at a point beyond the volume of plasma and before condensation of titanium dioxide thus produced is complete.
 26. A method according to claim 24 wherein the rutilising agent is an aluminum trihalide or a zirconium tetrahalide.
 27. A method according to claim 1 wherein the feed tube terminates within the central core of the downstream half of the volume of plasma. 